A conventional core charged particle beam imaging apparatus generally includes a primary charged particle beam source which generates a charged particle beam, a condenser lens to condense the generated charged particle beam, an objective lens to focus the charged particle beam into a fine probe, and a deflection unit to scan the fine probe over an area of a sample held on a stage. The charged particle beam probe (i.e., fine probe) interacts with the sample (at the scanned area) to excite secondary charged particles which carry information of local topography, material, and potential of the scanned area of the sample. A detection unit collects these secondary charged particles to form an image of the sample. One example of such a charged particle beam imaging technique is the scanning electron beam or scanning electron microscope (SEM).
In general cases, charging induced on the scanned area by the primary charged particle beam (i.e., fine probe) will accumulate if not released quickly. Though a proper level of charging of the sample is preferred to deliver a voltage contrast image, excessive and non-uniform charging will result in adverse effects on the image by distorting and/or defocusing the primary charged particle beam. This is especially a serious issue when the sample is insulative, such as with photolithography masks.
Currently, in some cases an approach to remove the sample surface charging is through use of a flood gun to intentionally project a separate charged particle beam, an optical beam or other electromagnetic radiation on the sample surface, so as to create a desired charging condition thereon. This operation is typically performed on a frame of image or wafer basis, i.e., it treats the sample surface in a large area at one time. However, with such an approach, when a subsequent imaging operation is performed, the established charging condition may have changed due to, for example, interaction of the regulated sample surface with the environment. In addition, this method requires an additional flood gun which has to be particularly designed to be integrated in the core imaging apparatus and operated in coordination with the imaging beam. This approach is disadvantageous and costly to both the system itself and its operation.
Another method of sample surface charging removal is through introduction of ionized gas molecules into the imaging room during imaging, so that charging on the sample surface can be neutralized by these charged particles. A potential drawback of this method is that the ionized gas molecules, although in small amount, can collide with and thus deviate the imaging beam, thereby damaging the quality of the obtained images. Further, this method also requires an additional gas ionizer and ionized gas molecule dispenser particularly designed to be integrated in the core imaging apparatus and operated in coordination with the imaging beam. This is again unfavorable from a cost point of view.
As mentioned above, the sample may be made of substantially insulative materials, such as, for example, with the mentioned photolithography masks. In such case, to release charging on the mask surface, physical contact with the sample to electrically couple it to ground is generally employed. However, as the imaging-induced charging is typically formed on the inspection area, which is a delicate pattern region on the mask surface, physical contact is undesired.
Accordingly, a method is needed for effectively releasing the sample surface charging without requiring the addition of extra mechanical hardware and/or operational software design to the core imaging apparatus.